Does the parenchyma mediate the ability of one airway to modulate the contraction of another
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If two airways are sufficiently close, the parenchymal distortion forces between them may interfere with their abilities to narrow. This mechanism may act to homogenize airway narrowing throughout the lung. We investigated this in a 2-dimensional computational model of a pair of airways embedded in parenchyma represented as a linear elastic continuum (Fig. 1a). Airway contraction was achieved by imposing an inward radial force on the hole boundaries and calculated using the finite element method. We determined how airway lumen area was affected as a function of the separation distance between their centers when only one airway contracted versus when both airways contracted equally. We found that airway contraction was identical under both conditions until the two airways came within about 2 uncontracted diameters of each other (Fig. 1b) at which point a contracting airway narrowed less if its companion airway also narrowed. We also found that when airways were close they narrowed more than when they were far apart, no doubt due to the reduced parenchymal interdependence forces caused by the hole representing a nearby companion. These model results suggest that airway-parenchymal interdependence will only significantly affect the heterogeneity of airway narrowing if the airways are, on average, within about 2 diameters of each other, provided that the parenchyma behaves like an elastic continuum.Keywords:
Parenchyma
Small airways
Lumen (anatomy)
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Parenchyma
Transpulmonary pressure
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Complex flow patterns exist within the asymmetric branching airway network in the lungs. These flow patterns are known to become increasingly heterogeneous during disease as a result of various mechanisms such as bronchoconstriction or alterations in lung tissue compliance. Here, we present a coupled model of tissue deformation and network airflow enabling predictions of dynamic flow properties, including temporal flow rate, pressure distribution, and the occurrence of reverse flows. We created two patient-specific airway geometries, one for a healthy subject and one for a severe asthmatic subject, derived using a combination of high-resolution CT data and a volume-filling branching algorithm. In addition, we created virtually constricted airway geometry by reducing the airway radii of the healthy subject model. The flow model was applied to these three different geometries to solve the pressure and flow distribution over a breathing cycle. The differences in wave phase of the flows in parallel airways induced by asymmetric airway geometry and bidirectional interaction between intra-acinar and airway network pressures were small in central airways but were more evident in peripheral airways. The asthmatic model showed elevated ventilation heterogeneity and significant flow disturbance. The reverse flows in the asthmatic model not only altered the local flow characteristics but also affected total lung resistance. The clinical significance of temporal flow disturbance on lung ventilation in normal airway model is obscure. However, increased flow disturbance and ventilation heterogeneity observed in the asthmatic model suggests that reverse flow may be an important factor for asthmatic lung function.
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Abstract Accurate knowledge of the airway geometry is needed when constructing physical models of the airway tree and for numerical modeling of flow or sound propagation in the airways. Human and animal experiments are conducted to validate these models. Many studies documented the geometric details of the human airways. However, information about the geometry of pig airways is scarcer. Earlier studies suggested that the morphology of animal airways can be significantly different from that of humans. The objective of this study is to measure the airway diameter, length and bifurcation angles in domestic pigs using computed tomography. In this study, lungs of six pigs were imaged, then segmentation software tools were used to extract the geometry of the airway lumen. The airway dimensions were measured from the resulting 3‐D models for the first 24 airway generations. Results showed that the size and morphology of the airways of the six pigs were similar. The trachea diameters were found to be comparable to the typical human adult, but the diameter, length and branching angles of other airways were noticeably different from that of humans. For example, pig airways consistently had an early branching from the trachea that feeds the top right lung lobe and precedes the main carina. This branch is absent in the human airways. The results suggested that the pig airways geometry may not be accurately approximated by human airways and this approximation may contribute to increasing the errors in computational models of the pig chest.
Lumen (anatomy)
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▪ Abstract The dimensions, composition, and stiffness of the airway wall are important determinants of airway cross-sectional area during dynamic collapse in a forced expiration or when airway smooth muscle is constricted. Under these circumstances, airway caliber is determined by an interaction between the forces acting to open the airway (parenchymal tension and wall stiffness) and those acting to close it (smooth-muscle force and surface tension at the inner gas-liquid interface). Experimental measurements and theoretical models of the airway tube law (relationship between cross-sectional area and transmural pressure) are presented. Data are presented for the elastic properties of the wall tissue. Simulations of airway constriction in normal and asthmatic airways are discussed. To the extent possible, comparisons are presented between the various models and existing experimental data.
Constriction
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An inverse model consisting of two elastic compartments connected in series and served by two airway conduits has recently been fit to measurements of respiratory impedance in obese subjects. Increases in the resistance of the distal conduit of the model with increasing body mass index have been linked to peripheral airway compression by mass loading of the chest wall. Nevertheless, how the two compartments and conduits of this simple model map onto the vastly more complicated structure of an actual lung remain unclear. To investigate this issue, we developed a multiscale branching airway tree model of the respiratory system that predicts realistic input impedance spectra between 5 and 20 Hz with only four free parameters. We use this model to study how the finite elastances of the conducting airway tree and the proximal upper airways affect impedance between 5 and 20 Hz. We show that progressive constriction of the peripheral airways causes impedance to appear to arise from two compartments connected in series, with the proximal compartment being a reflection of the elastance of upper airway structures proximal to the tracheal entrance and the lower compartment reflecting the pulmonary airways and tissues. We thus conclude that while this simple inverse model allows evaluation of overall respiratory system impedance between 5 and 20 Hz in the presence of upper airway shunting, it does not allow the separate contributions of central versus peripheral pulmonary airways to be resolved.
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Closure (psychology)
Bistability
Hysteresis
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